Lyme Disease Spreads in Midwest USA.

A recent study offers a look at Lyme disease in Illinois and suggests that deer ticks and the Lyme disease bacteria they host are more adaptable to new habitats than previously appreciated.

Led by researchers at the University of Illinois, the study gives an up-close view of one region affected by the steady march of deer ticks across the upper Midwest.

The spread of deer ticks began in Wisconsin and Minnesota, and is moving at a pace of up to two counties a year in Illinois and Indiana.

Today the deer tick is established in 26 Illinois counties, up from just eight in 1998, said Illinois Department of Public Health entomologist Linn Haramis. Reports of human Lyme disease cases in the state have more than tripled in the same period, he said.

“We’ve had several years in a row where we’ve had over 100 cases, up from about 30 per year more than 10 years ago,” Haramis said. “It’s not a huge increase, but it’s been steady and there’s an upward trend.”

Deer ticks are known to do best in forested areas, where they can readily move from small mammals (which provide their first meal) to moist leaf litter on the forest floor, and then to deer, on which they mate.

Deer ticks do not pick up the Lyme infection from deer, said Jennifer Rydzewski, who completed her master’s degree with the study in the department of natural resources and environmental sciences at the University of Illinois.

“The deer tick will feed on a variety of mammals, birds and even reptiles,” she said. “But Borrelia burgdorferi, the bacterium that causes Lyme disease, replicates really well within white-footed mice, so white-footed mice are the main reservoir that passes that bacterium on to the immature ticks that are feeding on it.”

White-footed mice also are forest dwellers. Prior to the new study, little was known about whether, or how, Lyme disease persists in other habitat types.